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Convert blower motor for ductless ops
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zz28zz:
I got a couple of air handler blower motors after replacing the home a/c sys. Wired one up to run on high speed. Connected it thru a amp meter to 120V. Motor is rated at 7.2A on high speed. My amp meter is reading 10A and every 20 secs or so, it would hiccup and pull ~11A for a half sec or so. Figured it was shot, then A/C guy says it needs duct work attached for it to run right. Started covering the outlet with a metal plate to create the back press it needs. Sure enough amperage dropped to 6A after covering abt 1/4 of the opening. Left it running for a few hours and it sounds happy. A/C  guy also mentioned he had ran one with no duct-work and it burned up after a few hours.

Started thinking abt the current and voltage peaks being out of phase. I'm assuming since a motor is inductive, if I increase the load on the motor (by partially blocking the outlet), the inductance will increase. The run capacitor is sized with a certain load assumed on the motor. If I run the fan with no blockage of the air, there's less load, so there's less inductance and now the run cap is too big.

Am I on the right track here? Will motor be happier with less capacitance when run with no ductwork?
amyk:
What you've described is very unusual for an induction motor; they're usually happiest to run at no load with minimum current draw, so I suspect the effect here may be due to something else, e.g. maybe the bearings are worn enough that the rotor is not directly centered in the stator (which will cause excessive current draw) and it needs the air pressure from the fan to push it into the correct place when running.
james_s:
It's possible that you have a bad bearing that is ok when run with some load on it but with the output wide open the motor spins up faster and/or loading on the shaft changes and allows it to float around in the worn bearing. Can you try removing the blower wheel and running the motor with no load?
Zero999:

--- Quote from: zz28zz on May 19, 2019, 05:33:41 am ---Started thinking abt the current and voltage peaks being out of phase. I'm assuming since a motor is inductive, if I increase the load on the motor (by partially blocking the outlet), the inductance will increase. The run capacitor is sized with a certain load assumed on the motor. If I run the fan with no blockage of the air, there's less load, so there's less inductance and now the run cap is too big.
--- End quote ---
Well no, in the case of a fan blocking the outlet reduces the load on the motor. This the same for a vacuum cleaner. Blocking the tube will reduce the air flow, so there is less force acting on the rotor, thus reducing the current consumption: notice how the motor turns faster? It's because there's less load on it to slow it down. Of course there is a limit, because it fit's blocked too much, the motor might not be able to adequately cool itself and in the case of a vacuum cleaner, with a universal motor running at light loads can damage the bearings from overspeed, but it's plausible that allowing full air flow could cause overheating.
Simon:
As zero999 said. It is a common misconception that is even held by my work collegues in engine cooling and air conditioning that blocking a fan makes it work harder, it does not.

The work a fan does is to rotate the impellor (flywheel) and move air, if you take the air movement out by blocking either end it will reduce the work load. It's a fan not a piston compressor. Fans follow fan laws. The flow is directly proportional to the speed and the pressure follows a square law. So if you double a fans speed it will move twice the air and be capable of 4 times the pressure, this means that the power required will be 8 times (2x4). This means that slowing the fan down only slightly will greatly reduce power consumption. For example running at 80% speed will drop the power draw to 1/2. Speeding fans up is a bad way to increase air flow because of this, it is more efficient to use a bigger fan and run at the same speed.

Pushing air through a duct also has laws. If you push air through a duct there will be pressure drop, if you want to double the flow you will get 4x the pressure drop but happily if you double the speed of your fan it produces 4x the pressure. So I would guess that the motor was given and impellor that was too much load for it in free air safe in the knowledge that once put into the AHU it would be restricted anyway so move less air because of the pressure drop in the pipework.

What sort of motor is it? I have played with a brushless automotive blower that runs at a constant speed no matter the load. Holding it down on the bench so that air cannot flow it only uses 2A, but as soon as I lift it off it uses 16A as it now has work to do other than turn a plastic wheel.
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